With the discovery of a number of cancer-related genes in recent years, revelation of the mechanism of malignant transformation has been slowly began. It has been shown that many cancers are certain genetic diseases caused by gene abnormalities. Since abnormal mutant cells originate naturally from normal cells, they are not easily recognized as foreign bodies in the living body, and their biological or biochemical properties are not so different, unlike the case of infectious diseases caused by the invasion of other organisms.
In consequence, though various anticancer drugs, therapeutic methods and reagents have been developed, they exert influences upon not only cancer cells but also normal tissues and normal cells because of their low selectivity for cancer cells, so that, in the present situation, the use of such drugs is strictly restricted due to their side effects no matter how they are effective.
On the contrary, controlling methods of the expression of specific genes have recently been developed making use of nucleic acids and their derivatives. Since cancers are genetic diseases as described above, cancer cell-specific effects can fully be expected by such expression controlling methods when a specific gene, particularly a malignant transformation factor, is used as the target.
One of such means is antisense method. In this method, a short oligonucleotide (approximately 15 to 30 base length) having a nucleotide sequence complimentary to a nucleotide sequence of a gene to be used as a target is introduced into cells to effect inhibition of transcription or translation of the malignant transformation gene. The antisense method is roughly divided into antigene method and antimessenger method depending on the target molecules.
The antimessenger method uses messenger RNA as the target. In this method, an oligonucleotide having a nucleotide sequence complementary to a target single-stranded RNA molecule is prepared to form a double-stranded chain by the Watson-Crick bonding, thus effecting inhibition of splicing or translation of the target RNA or its degradation with a double strand-specific RNA degrading enzyme. Unlike the case of the antigene method which will be described later, this method can use all RNA molecules as the target. Therefore, studies on this method have been reported using as the target various oncogenes such as c-myc gene (Nature, 328, 445-449, (1987)) and the like. However, since messenger RNA molecules are present in cells in a relatively large amount (especially in the case of malignant transformation factors), the amount of antisense molecules required for exerting influence upon cells also becomes large. Therefore, it is considered that even if its effect is recognized at the level of cultured cells, its clinical application is difficult for the present.
In the antigene method, on the contrary, an antisense molecule is wound round a double-stranded DNA chain of a target gene to form a triple-stranded chain. Because the amount of such target in cells is small, unlike the case of the antimessenger method whose target is messenger RNA, the antigene method which targets gene DNA seems to be a more suitable method for clinical application, when stability and the like of oligonucleotides in the living body are taken into consideration. Formation of the triple-stranded chain which is essential for the antigene method depends on the Hoogsteen bonding. It is known that one side of double-stranded DNA must have a continuous purine base sequence (homopurine/homopyrimidine sequence) for the formation of the Hoogsteen bonding.
Accordingly, the object of the present invention is to provide, making use of the antigene method, an oligonucleotide or a derivative thereof which acts selectively on cancer cells and is therefore useful as a pharmaceutical composition such as anticancer agent, a method for the prevention and treatment of cancers which uses the pharmaceutical composition, and a method for the inhibition of p120 gene transcription.